1. The Field of the Invention. This invention relates to a picture element (pixel) artifact or blemish filter and more particularly to such a filter designed especially for use in charge-coupled device video cameras.
2. The Background Art. A charge-coupled device (CCD) is comprised of an array of semiconductor elements typically formed on an integrated circuit chip, in which each element can store an electrical charge and the charges can be transferred from one element to the next to an output location or terminal. CCD's have gained widespread use in video cameras as a component for storing video images, the charges stored in the semiconductor elements each representing a picture element (pixel) in the image. The quantity of charge in each semiconductor element, at an instant in time, represents the amount of light in the corresponding pixel of the video image. In other words, the stored charges in the CCD represent a stored video image which can be read from the CCD by transferring the charges, as a series of electrical pulses called the video signal, to an output location. See, for example, Curry, Thomas S., Dowdey, James E. and Murry, Robert C. Christensen's Physics of Diagnostic Radiology, 4th Edition, Lea and Febiger, 1990, Chapter 13.
CCD video cameras, in addition to becoming the dominantly used consumer video camera, are increasingly being used in the imaging chain of fluoroscopic imaging systems. However, acceptance has been slowed in part due to image artifacts or blemishes from pixel non-uniformities. These artifacts or blemishes may take the form generally of either variations in gain (sensitivity) or offset (leakage or dark current) of individual pixels. The lighter (brighter) form of artifact or blemish is especially offensive to the user since they appear as scattered small white dots in a fluoroscopic image, and become more obvious as the temperature increases with continued use of the fluoroscopic equipment.
Various image processing approaches have been proposed for eliminating the pixel artifacts including the storing in digital memory of each pixel information and also storing the offset and gain parameters for each pixel, and then using these parameters to correct the video signal on a pixel (pulse) by pixel (pulse)basis during production of the video image. This approach is costly and complicated and therefore generally not favored. Another proposed approach is to use a "median filter" to detect non-uniform or variant pixel values and replace them with values determined by surrounding pixels. This approach also requires costly and complicated video processing circuitry.